In recent years, image forming is performed on a sheet by an image forming device or the like sometimes in such a manner that cut marks that indicate a cutting position are added to image data for forming an image. This makes easier the sheet cutting task with a sheet cutter after image forming and improves working efficiency.
Cut marks are usually put on each page. That is, in a case of forming an image on both sides of a sheet, cut marks are attached to both sides (for example, see Patent document 1). Patent Document 1 discloses a document making device that performs printing on a single sheet such as a name card or a post card. Specifically, a side of a sheet is used as it is, as a reference side in printing, reducing the number of times of cutting the sheet. By the document making device disclosed in Patent Document 1, cut marks are automatically printed on the pages of both sides of a sheet.
However, in case of printing cut marks on both sides of a sheet, if there is a shift and difference in the position relationship between cut marks on the front side and those on the back side, the marks defining a cutting position, cut marks printed on a side of a sheet may remain when cutting is performed referring to cut marks printed on the other side. If unnecessary cut marks that are images other than image data remain on a sheet, a problem of degrading the image quality is caused.
Further, in Patent Document 2, with respect to the printing field and particularly to an imposition method of carrying out multiple imposition processing of design images in order to prepare printing masters and a ledger sheet design system using such a method, an imposition method has been disclosed that has a setting process to set the parameters of positioning marks. In a point of specific technical view, it has been disclosed that a type is to be set on a register mark setting screen to determine as to whether register marks are to be put on the front side or the back side. In this technology, it is necessary for the user to select and designate whether or not to attach register marks, and hence the procedure is not simple or easy. In addition, in this technology, register marks are to be attached to the front side or the back side, and register marks are to be attached to each sheet when printing is performed on plural pages.
Patent Document 1: TOKKAI No. H9-109505
Patent Document 2: TOKKAI No. H8-241399
However, researches by the inventors proved that a special problem occurs when cut marks are attached by an image forming apparatus that form images using the electro-photographic method. That is, the image forming process in the electro-photographic method includes a heat-and-pressure fixing process that fixes a toner image on a recording sheet by applying heat and pressure. When the sheet is heated during this heat-and-pressure fixing process, the moisture contained in the sheet evaporates due to being heated and also the fiber structure constituting the sheet becomes soft. Thus, a phenomenon occurs that the sheet shrinks after the heat-and-pressure fixing process.
When forming images on both sides of a sheet, even if an image is formed on a first side in a predetermined size, the shrinking phenomenon of the sheet slightly-reduces the size of the sheet and that of the formed image. Next, in image forming on the second side, an image is formed under the same conditions on the second side of the sheet that is the back side opposite to the first side, wherein the front side is in a reduced size and has the image in a reduced size. In this situation, the size of the image on the first side will be smaller than that of the image on the second side, which is caused by the shrinkage of the sheet described above. As a consequence, there will be a gap between the positions of the images on the first and second sides. In case of forming images as cut marks, shrinking of the sheet during the heat-and-pressure fixing process essentially creates a gap between the positions of the images on the first and second sides, even if image forming is attempted to be performed on the same position on the front and back sides. When cutting processing is performed with reference to the cut marks on either the front side or the back side with a gap between the positions of cut marks on the front side and the back side, it is possible that the cut marks that were not referred to remain on the sheet after cutting, and hence the image quality goes down.
Further, in the case of forming images on plural sheets by the electro-photographic method, a transfer process that transfers an image onto a sheet conveyed by conveying rollers is included, wherein variation in the conveyance and transfer timing of the respective sheets can not be avoided, which makes it difficult to form images on the same position of the respective sheets. Accordingly, variation is caused in the positions of image forming on the respective plural sheets. Accordingly, in case of forming images as cut marks, the above described variation essentially creates gaps between the positions of the images, even if image forming is attempted to be performed on the same position on the respective sheets. In the case where cut marks are attached to the respective sheets with gaps between the positions thereof, if a cutting processing is performed with reference to the cut marks on the sheet on the top surface of the stack of the sheets, cut marks on the sheets other than the one on the top surface may remain after cutting, which degrades the image quality.